The present invention relates to a collector arrangement for a solar energy heating system in which a plurality of cell elements cooperate with pipes for the passage of a heat exchange medium.
There is no doubt that there is only a limited supply of industrially extractable fossil energy sources, such as coal, oil, oil sand, oil shale, and natural gas. This applies more particularly to the present main energy source, oil. Even when taking account of new oil fields, it is excepted that oil supplies will be exhausted in about 50 years. Certain internationally recognized research institutes have in fact made even less favorable forecasts. However, even these pessimistic forecasts must not hide the fact that on the basis of the law of supply and demand the oil shortages which will occur much earlier will lead to considerable price rises. The effects of such oil rises for our economy became very apparent after the last oil crisis.
Attempts are therefore being made to find new independent energy sources. The main object of these efforts is to develop a maximum number of substitute energy sources, using these whenever possible in place of oil. World-wide statistics show that a large proportion of existing oil supplies is being used for heating purposes.
However, for this purpose solar energy constitutes an ideal substitute energy source. Quite apart from the fact that this energy is available free and in unlimited quantities, it is characterized by having no harmful effects on the environment. A further important advantage is that it can be used on a completely decentralized basis. However, it must be remembered that this energy can only be obtained by day and, in part, only with direct solar radiation. Account must also be taken of the fact that solar energy only has a relatively low intensity, particularly with clouded skies, when only the much less diffuse rays can be used.
A number of commercially usable systems for using solar energy for heating purposes are already known. In particular, a so-called solar cell system has been adopted, a distinction being necessary between flat cells and focussing cells.
In the case of flat cells, the basic construction is always essentially the same. A generally metallic flat body is provided with a radiation-absorbing surface which generally consists of black lacquer or some similar material. Pipes or ducts are placed in or on the body. Through the pipes or ducts circulates a heating exchange medium which conveys away the heat trapped by the absorbing surface. This heat can be used either directly or indirectly for heating purposes.
Since, according to the Stefan-Boltzmann law, each black body also emits thermal radiation whose intensity rises to the fourth power of its temperature, the cells are covered with one or more layers of glass or plastic. These glass or plastic layers are not transparent for the wavelength range of the rays emitted by the cell, so that the partly reflected rays are largely absorbed by the glass or plastic layers, where they are converted into heat, leading to the so-called hothouse effect.
The back of the cell is provided with a sufficiently thick insulating layer, so that only very small losses occur here.
To make the ratio between absorption and emission more favorable, in more sophisticated flat cells the black surface is replaced by a so-called selective surface. Selective surfaces have the advantage that the solar radiation is absorbed very well and emission is very small.
Flat cells have a number of advantages. Thus, they are able to convert even diffuse radiation into thermal energy. In addition, there is a good efficiency up to a heating medium temperature of 60.degree. C. (Celsius). Furthermore, the flat cells are relatively inexpensive and simple to install.
However, reference must also be made to certain of the disadvantages of flat cells. A particular disadvantage is the poor efficiency in the high temperature range, temperatures above 100.degree. C. (Celsius) being very difficult to obtain. Even in the case of optimum alignment of the cells relative to the sun, i.e. its position at midday, there is a particularly strong reflection from the flat covering plates in the morning and evening, due to the very acute angle of incidence, so that efficiency drops. This is very disadvantageous because, other than at night, it is particularly in the morning and evening that thermal energy is required. It is also very difficult to obtain an air-tight seal for the space between cell and covering layer. Moist air frequently enters this space and leads to fogging of the panes of glass, so that efficiency drops. The ideal solution would be a high vacuum in this space. However, due to the relatively large areas, even a low vacuum would cause the panes of glass to break. To obtain good efficiency, the cell surface must be aligned as precisely as possible with the mean solar position. However, in the case of house roofs which do not have this optimum position and inclination, installation is difficult or efficiency is low. For numerous reasons it is unlikely that a sail-like installation of the cells would be permitted. In addition, as the cells are generally only made in certain sizes, it is difficult to adapt them to particular roof shapes.
In the case of focussing solar cells, the incident solar radiation is focussed onto a point, line, or surface my means of an optical system, e.g. a mirror or lens system. In the case of solar cells for heating purposes, generally cylindrical-parabolic mirrors are used in which the incident rays are concentrated on a line. Rotationally symmetrical parabolic mirrors are less frequently encountered.
Cylindrical parabolic mirrors generally are made from glass, to the back of which is applied a thin silver coating. This coating reflects the incident rays onto the focal line of the parabolic mirror.
According to a further variant, the parabolic mirror is made from highly polished metal.
The absorber is also located in the focal line of the parabolic mirror. The absorber is generally constituted by black metal tubes or glass-covered black metal tubes, glass tubes with a black licquid which is simultaneously used as a heat carrier medium, or special metal profiles surrounded by a glass tube. A number of telescoped glass tubes could also be used, one being provided with a selective absorber coating and the underlying glass tube serving as a supply and discharge tube for the heat carrier medium.
An important feature of focussing cells is the concentration factor. This factor C forms the ratio of the admission surface of the cell to the absorber surface. The higher the factor C, the higher the temperature to which the carrier medium can be heated.
Advantages of focussing cells are, inter alia, that very high temperatures can be obtained as a function of the concentration factor. In the case of indirect further use, the temperature level is a measure for good efficiency. It is also advantageous that the emission and convection losses are much lower than with flat cells, due to the small absorber surface compared with the admission surface.
It is disadvantageous that focussing cells only operate with direct solar radiation, and must therefore follow the sun, which requires an additional mechanism. In addition, they are expensive to maintain and can only be installed on suitable roofs, preferably flat roofs. It is very difficult to install them on inclined house roofs. Their wind pressure sensitivity is a further disadvantage. They are also relatively costly, sizes cannot be varied as desired, due to their standard, and cannot be individually adapted readily to special requirements.